Holding a flexible plastic tape measure perpendicular to the ground, measure around your horse's heart girth--starting about 4 inches behind his front leg at the girth area, up over the highest point of his withers and around his barrel. Enter the number of inches in the first box above, marked "heart girth." Then, with the tape parallel to the ground, measure his body length from his point of shoulder to his point of buttock. Enter that number in the second box, marked "body length." Finally, select your horse's age from the drop down menu and click "calculate." Voila!
[In case you would rather do it the long way, the formula for a horse older than one year is (heart girth x heart girth x body length)/330=estimated body weight in pounds. For a weanling, divide by 280, and for a yearling, divide by 301.]
For more information about estimating your horse's weight, see the June 2008 issue of Practical Horseman magazine.